Hi Kexin,
Try running:
fcm branch-create --bob -k 7783 vn12.0_weiyu_contrail_practicle fcm:um.x_br/dev/weiyuzhang/vn12.0_contrail_Chen2012@119756
You were using fcm:um.xm_br which refers to the read-only mirror repositories on PUMA2 not the writable repositories on the Met Office Science Repository Service.
Depending on what exactly you are doing it is usually bettter to create a branch from the UM trunk (e.g. fcm:um.x-tr@vn12.0) and then fcm merge in the branch you wish to branch from (e.g. vn12.0_contrail_Chen2012@119756) rather than trying to do a branch-of-a-branch. Sometimes this can cause problems when the UM does the code extraction/merge.
Regards,
Ros